Bolton will serve as the 27th man for Sunday's doubleheader against the Reds, Jason Mackey of the Pittsburgh Post-Gazette reports.
Bolton had a May 29 appearance against the Giants in which he gave up eight earned runs while recording just one out, so that has really bloated his numbers (9.90 ERA, 2.80 WHIP) on the season. He will likely head back to Triple-A Indianapolis after the twin bill.